1. Field of the Invention
The present invention relates to an electron beam writing apparatus that has a holding mechanism for holding a mask at its back surface and irradiates the surface of the mask held by the holding mechanism with an electron beam thereby to write a desired pattern, and an electron beam writing method therefor.
2. Background Art
An upper surface holding mechanism for holding a mask upper surface has heretofore been adopted upon writing of each pattern on a mask by an electron beam writing apparatus. FIG. 7A shows a state in which a mask M is held by the upper surface holding mechanism. The upper surface holding mechanism is provided on an XY stage 104 movable in the direction orthogonal to the direction of an optical axis of an electron beam (not shown) emitted from an electronic optical lens barrel 101 equipped with an electronic optical system and comprises a clamp 102 having a top reference piece, for clamping the upper surface of the mask M and clamp pins 103 which are brought into contact with the back surface of the mask M coaxially with the top reference piece and urged upwardly by urging means provided outside the drawing, and has a structure in which the mask M is interposed between and held by the top reference piece of the clamp 102 and the clamp pins 103 (refer to, for example, a patent document 1).
Since the upper surface holding mechanism minimizes the contact with the mask M, cleanliness of the mask M can be maintained. Further, the upper surface holding mechanism suppresses an influence of an error in the surface of the mask M, which occurs due to the correction of the shape of the back surface of the mask M and is excellent in reproducibility.
Meanwhile, in order to meet a demand for miniaturization of circuit patterns of a recent semiconductor and the like, there is a need to enhance the resolution of an electron beam. To this end, an approximation of the electronic optical lens barrel 101 to the mask M is becoming very important. While the distance between the electronic optical lens barrel 101 and the mask M is now on the order of a few mm or so, the thickness of the top reference piece of the clamp 102 is also on the same order as that. Therefore, when the electronic optical lens barrel 101 is brought close to the mask M to meet the above demand as shown in FIG. 7B, it is becoming difficult to ensure space for the top reference piece of the clamp 102.
In order to solve such a problem, a back surface holding mechanism 105 for holding a mask M at its back surface, such as an electrostatic chuck may be used as a mask holding mechanism as shown in FIG. 8A. When, however, the back surface holding mechanism 105 is used, the thickness of the mask M has a tolerance of the order of 100 μm. Therefore, unlike the case where the upper surface holding mechanism described in FIG. 7A and 7B are used, a vertical position displacement of the surface of the mask M becomes manifest (refer to FIG. 8B).
When the vertical position displacement occurs, focus senility, a position displacement and the like occur upon actual writing even though a focal adjustment to the electron beam and a beam position adjustment are conducted by a mark table, thus causing a disadvantage that such influence is exerted on writing accuracy. If the heights of a mark table 106 and a mask M1 coincide with each other as shown in FIG. 9A by way of example, then writing can be conducted by an electron beam adjusted in advance by the mark table 106. However, when a vertical displacement g1 is taking place because a mask M2 is higher than a mark table 106 as shown in FIG. 9B and when a vertical displacement g2 is taking place because a mask M3 is lower than a mark table 106 as shown in FIG. 9C, there is a need to correct the influences of the vertical displacements.
Here, an electronic optical lens barrel is equipped with a focal adjustment mechanism for varying a focal height of an electron beam in a predetermined adjustable range. It is considered that the correction of the influence of a vertical displacement is performed by the adjustment of the focal height by the focal adjustment mechanism (refer to, for example, a patent document 2). While there is a need to set an adjustable range for the focal height to the order of 100 μm corresponding to a mask's thickness tolerance in this case, it is difficult to ensure such a large adjustable range.    [Patent Document 1] Japanese Patent Application Laid-Open No. Hei 10(1998)-55950    [Patent Document 2] U.S. Pat. No. 6,741,331